Progress has been made in recent years in the development of vehicles that are equipped with a seat belt device for protecting a vehicle occupant. Some vehicle seat belt devices are provided with a pretensioner for rapidly retracting a loose portion of the seat belt in emergency situations. A type of vehicle seat belt device in which the seat belt is vibrated in order to warn the occupant of an anticipated emergency condition in the vehicle is known as a technique for increasing occupant protection (see JP-A-2000-6759 and JP-A-2004-276896).
The vehicle seat belt device described in JP-A-2000-6759 or JP-A-2004-276896 is provided with an electric motor for retracting the loose portion of a seat belt during a vehicle emergency. When an emergency condition in the vehicle is predicted, the operation of the electric motor is intermittently repeated to cause forward rotation or backward rotation.
The seat belt is retracted when the electric motor rotates forward, and the tension of the belt therefore increases. The seat belt is unwound when the electric motor rotates backward, and the tension of the belt therefore decreases. Intermittently repeating the increase and decrease in belt tension causes the seat belt to vibrate with a constant force. A belt vibration can thus be used to alert an occupant and call the occupant's attention by intermittently tightening and loosening the level of restraint exerted on the occupant by the seat belt.
The restraining force of the seat belt experienced by the seated occupant varies according to the state of the occupant (occupant's body position, the type or thickness of clothing, and other characteristics) or the effect of the looseness of the seat belt, and “deviation” therefore occurs. The occupant can therefore feel that the restraining force is too small or too great when an emergency is predicted. When the restraining force felt by the occupant is too great, the rapid tightening of the seat belt that occurs each time an emergency condition is predicted is uncomfortable to the occupant. It is also difficult for the occupant to notice the vibration of the seat belt when the restraining force felt by the occupant is too small. The same is true when the seat belt is loose at the time an emergency condition is predicted. The restraining force felt by the occupant is more preferably optimized at all times in the prediction stage.
Furthermore, even when the restraining force felt by the occupant is appropriate, the occupant feels the impact of the seat belt when the momentary change in restraining force that accompanies belt vibration is sudden. Such a sensation of impact is irritating to the occupant, and should therefore be made smaller. When there is “deviation” in the momentary change in restraining force, the magnitude of the irritation felt by the occupant also changes. The change in the level of irritation should also preferably be small.
A potential therefore exists for improvement in order to call the attention of an occupant by imparting a belt vibration that is more suitable to the occupant.
There is therefore a need for a technique that is capable of more reliably alerting an occupant and eliminating the discomfort associated with the alert by vibrating a seat belt in a stage in which a vehicle emergency condition is predicted, regardless of the state of the occupant (occupant's body position, the type or thickness of clothing, and other characteristics) or the looseness of the seat belt. There is also a need for a technique whereby the occupant's attention can be called by imparting a more suitable belt vibration to the occupant.